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Acidification of the Coastal Ocean: Are deep waters of the Gulf of Maine already corrosive to
pteropods?
Z. Aleck Wang, Gareth Lawson, and Amy Maas
Final Report
Background and Objectives – As a result of increases in atmospheric carbon dioxide (CO2),
the ocean is taking up extra CO2 and becoming more acidic, in a process referred to as ocean
acidification (OA, Fig. 1). Certain coastal regions, such as the upwelling system along the U.S.
West Coast, are more susceptible to the effects of ocean acidification than others, because their
waters are naturally low in pH and saturation of aragonite, but higher in CO2 concentration, at least
at some times of year. Aragonite is a mineral type of calcium carbonate, and its level in seawater
(measured by its saturation state) is an important indicator for shellfish growth of their shells.
Recent evidence has suggested that the US northeast coastal waters, including the Middle Atlantic
Bight (MAB) and Gulf of Maine (GoME), are more susceptible to ocean acidification than has
previously been thought. Aragonite saturation state (ΩA) in the US Northeast coastal waters on
average is close to a border line value of 1, below which seawater chemical conditions would favor
dissolution of aragonite shell (corrosive to aragonite, Fig. 1). If ocean acidification continues, US
northeast coastal waters may be the first along
the East Coast that experience corrosive
condition to aragonite, thus the first to ‘feel’
OA effects.
Thecosome pteropods are a group of
aragonite shell-forming zooplankton that may
be among the first pelagic organisms to
experience the consequences of ΩA undersaturation in the GoME (Fig. 1). Limacina
retroversa is an important pteropod species in
ΩA > 1
ΩA = 1
ΩA < 1
temperate latitudes of the North Atlantic and
is by far the most common pteropod species
in the Gulf of Maine, where it is a food item
for a number of important fisheries species
and other higher predators.
This interdisciplinary project aimed to
Fig. 1. Schematics of ocean acidification and its impact
on pteropods. Inset: Limacina retroversa, a species of
assess seasonal variations of the CO2 system
shelled pteropod endemic to the Gulf of Maine.
in the deep GoME and the associated impacts
on thecosome pteropods. The main objectives
of this project were to: (1) investigate if deep waters of the GoME are already seasonally undersaturated with respect to aragonite saturation state and (2) quantify seasonal patterns in the
abundance of the most common thecosome pteropod in the GoME, Limacina retroversa, and
examine the impacts of potential under-saturation of aragonite on its vertical distribution.
Results – To achieve the objectives of the project, we conducted a series of seasonal research
cruises in May, August, and October of 2013 and January and April of 2014 on the R/V Tioga to the
deep portions of Wilkinson Basin (~300m) in the GoME (Fig. 2). At each station, we collected water
samples for measurements of the seawater CO2 chemistry, determined pteropod vertical distributions
using depth-stratified nets, and captured live pteropods for physiological experiments (as part of other
funded work). The first two cruises were conducted with COI support. Initial findings from those two
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cruises led to additional funding from the Pickman
Foundation to support carbonate chemistry sampling and
analysis on three additional cruises conducted as part of an
NSF-funded project examining pteropod physiology.
The results analyzed so far indicate that near
Gulf of Maine
bottom waters of the Wilkinson Basin in the GoME are
already experiencing seasonal corrosive condition with
respect to aragonite at the end of May (Fig. 3). In other
words aragonite shells of pteropods and other shellbearing organisms can dissolve in these waters, in the
absence of any physiological compensation
mechanisms. This region of aragonite undersaturation
corresponded to the bottom nepheloid layer, which has
the maximum particle concentration in the water
Fig. 2. Station map in the GoME
column that may result from resuspension of sediment.
This condition occurs at the end of spring phytoplankton bloom, where a pronounced subsurface
primary production maximum occurred (Fig. 3). It suggests that aragonite undersaturation near
bottom may be a result of decomposition of freshly settled dead materials from surface and
subsurface phytoplankton bloom. Organic matter is broken down by microbial activity which
consume oxygen, release CO2, and lower pH and hence aragonite saturation.
Interestingly, such a corrosive condition did not occur during the other three seasonal
sampling cruises, especially for August and October when primary production was high near
surface. Previous studies on bioparticles in the Wilkinson Basin suggests that aragonite shells are
missing from sediment traps and aragonite particles in the water column decrease rapidly with depth,
all of which seems to provide the evidence that near bottom waters should be corrosive to aragonite
shells for much of the annual cycle. This project thus finds an interesting research question that
warrants future studies.
Depth-stratified net sampling found that thecosome pteropods were most abundant in nearsurface waters and absent from the deepest portions of the water column where aragonite saturation
states were lowest (e.g., Figure 4). It is noteworthy that maximum abundance was found at shallow
depths given that sampling occurred during the daytime, when many zooplankton, including
pteropod species in other
regions, tend to migrate to
large depths in order to
avoid visual predators.
Limacina retroversa being
found near the surface
suggests it may avoid any
deleterious consequences of
under-saturation at depth,
but as a consequence may be
exposed to greater predation
threat.
Fig. 3. Depth profiles of aragonite saturation states and total CO2
(DIC) concentrations at Station 2 over different seasons.
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0 - 25 m
Depth Interval
25 - 50 m
50 - 75 m
75 - 100 m
100 - 150 m
150 - 200 m
200 - 230 m
230 - 255 m
0.01
0.1
1
10
100
Pteropod Abundance (#/m3)
Fig. 4. Vertical distribution of Limacina retroversa from
depth-stratified net sampling at Station 2 in May, 2013.
The current study so far demonstrates
that undersaturation of aragonite does occur
near bottom waters in the Wilkinson Basin of
the GoME. However, we still have limited
knowledge on the controlling mechanisms of
such undersaturation, as well as the likely
biological consequences to pteropods (and
pteropod predators) and other calcifying
organisms. We do not know how widespread
the undersaturation is in the GoME. As the
region is famous for its profitable shellfish
industry, the implication for our finding is
significant not only to biology and the
ecosystem but to regional socio-economics. It is
expected that undersaturation of aragonite in
the GoME would become more prevalent as
ocean acidification continues.
Leverage, Education, and Outreach – This project has generated important results that
allowed us to secure another grant from the New Hampshire Charitable Foundation to continue
seasonal sampling in Wilkinson Basin. We are planning to submit another proposal to the National
Science Foundation (NSF) to study the mechanisms of controlling the aragonite saturation state in
the region. The importance of this project is also reflected in several aspects: (1) The results
described here on seasonality in carbonate chemistry are providing key insight into the acclimation
of pteropods to seasonality that is studied by a related NSF project;(2) This project has provided
training and research opportunities for two postdoctoral scholars, three WHOI-MIT Joint Program
graduate students, and multiple student interns; (3) Lawson and Wang are featured in an exhibit on
pteropods and OA at the Sant Ocean Hall of the Smithsonian’s National Museum of Natural History
and Lawson participated in the Museum’s “The Scientist Is In” program; (4) Project-related
materials have been communicated to a K-12 audience and the general public via the WHOI “Image
of the Day” site. The Lawson lab also contributed multimedia for the PBS Learning Media by
KCTS in Seattle: https://www.youtube.com/watch?v=MzpmUHdwLr0.
Sampling in the GoME on R/V Tioga.
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